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    <div class="example" id="timeouts">
      <h2><a href="./">Go by Example 中文版</a>: 超时处理</h2>
      
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            <p><em>超时</em> 对于一个需要连接外部资源，
或者有耗时较长的操作的程序而言是很重要的。
得益于通道和 <code>select</code>，在 Go 中实现超时操作是简洁而优雅的。</p>

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            <a href="http://play.golang.org/p/jtL21Ki_X3b"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>

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          <div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
    <span class="s">&quot;fmt&quot;</span>
    <span class="s">&quot;time&quot;</span>
<span class="p">)</span>
</pre></div>

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          <div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>

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            <p>在这个例子中，假如我们执行一个外部调用，
并在 2 秒后使用通道 <code>c1</code> 返回它的执行结果。</p>

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          <div class="highlight"><pre>    <span class="nx">c1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
    <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
        <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
        <span class="nx">c1</span> <span class="o">&lt;-</span> <span class="s">&quot;result 1&quot;</span>
    <span class="p">}()</span>
</pre></div>

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            <p>这里是使用 <code>select</code> 实现一个超时操作。
<code>res := &lt;- c1</code> 等待结果，<code>&lt;-Time.After</code> 等待超时（1秒钟）以后发送的值。
由于 <code>select</code> 默认处理第一个已准备好的接收操作，
因此如果操作耗时超过了允许的 1 秒的话，将会执行超时 case。</p>

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          <div class="highlight"><pre>    <span class="k">select</span> <span class="p">{</span>
    <span class="k">case</span> <span class="nx">res</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c1</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">res</span><span class="p">)</span>
    <span class="k">case</span> <span class="o">&lt;-</span><span class="nx">time</span><span class="p">.</span><span class="nx">After</span><span class="p">(</span><span class="mi">1</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">):</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;timeout 1&quot;</span><span class="p">)</span>
    <span class="p">}</span>
</pre></div>

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            <p>如果我们允许一个长一点的超时时间：3 秒，
就可以成功的从 <code>c2</code> 接收到值，并且打印出结果。</p>

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          <div class="highlight"><pre>    <span class="nx">c2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">chan</span> <span class="kt">string</span><span class="p">,</span> <span class="mi">1</span><span class="p">)</span>
    <span class="k">go</span> <span class="kd">func</span><span class="p">()</span> <span class="p">{</span>
        <span class="nx">time</span><span class="p">.</span><span class="nx">Sleep</span><span class="p">(</span><span class="mi">2</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">)</span>
        <span class="nx">c2</span> <span class="o">&lt;-</span> <span class="s">&quot;result 2&quot;</span>
    <span class="p">}()</span>
    <span class="k">select</span> <span class="p">{</span>
    <span class="k">case</span> <span class="nx">res</span> <span class="o">:=</span> <span class="o">&lt;-</span><span class="nx">c2</span><span class="p">:</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">res</span><span class="p">)</span>
    <span class="k">case</span> <span class="o">&lt;-</span><span class="nx">time</span><span class="p">.</span><span class="nx">After</span><span class="p">(</span><span class="mi">3</span> <span class="o">*</span> <span class="nx">time</span><span class="p">.</span><span class="nx">Second</span><span class="p">):</span>
        <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;timeout 2&quot;</span><span class="p">)</span>
    <span class="p">}</span>
<span class="p">}</span>
</pre></div>

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            <p>运行这个程序，首先显示运行超时的操作，然后是成功接收的。</p>

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          <div class="highlight"><pre><span class="gp">$</span> go run timeouts.go 
<span class="go">timeout 1</span>
<span class="go">result 2</span>
</pre></div>

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      <p class="next">
        下一个例子: <a href="non-blocking-channel-operations">非阻塞通道操作</a>
      </p>
      
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